The difference between dark chocolate and milk chocolate is in the ingredients.

These are the minimum ingredients needed to make the chocolate.

Cocoa mass
Cocoa mass is created by grinding cocoa beans and is an important raw material for chocolate.

Cocoa butter
Cocoa butter is extracted from cocoa beans. The cocoa beans are ground and cocoa butter is pressed from this mass.
Some chocolate makers choose to add extra cocoa butter to the cocoa mass. How is no rule for it has everything to do with personal preference and the style of the chocolate maker. Too little cocoa butter can create a dry mouthfeel, while too much cocoa butter can make the chocolate oily, slimy and lacking in flavour.

Sugar
Pure bleached cane sugar is the most common choice because it has no flavour. Natural or organic cane sugar is often preferred, but has the disadvantage that its own flavour mixes with the flavours of the chocolate. Alternative sugars such as coconut blossom sugar are becoming increasingly popular, but these all have their own distinctive flavour that changes the overall flavour profile of the chocolate.

Dark chocolate
Dark chocolate must contain a minimum total of 35% of cocoa mass and cocoa butter.

Milk chocolate
Milk chocolate must contain a minimum total of 25% of cocoa mass and cocoa butter.